(a) Technical Field
The present invention relates to a torque control method for a hybrid electric vehicle (HEV), which can improve driving performance and power performance, in the event of integrated starter-generate (ISG) failure, by performing a hydraulic control for a clutch and calculating a driver request torque and a transfer torque from the clutch to a motor to compensate the drive request torque to a desired level.
(b) Background Art
A hybrid electric vehicle (HEV) employs a motor drive source as an auxiliary power source as well as an internal combustion engine to provide a reduction in exhaust gas and an improvement in fuel efficiency. As shown in FIG. 1, the HEV has a structure in which an engine 10, a motor 20, and an automatic transmission 30 are arranged in a line.
In detail, the engine 10, the motor 20, and the automatic transmission 30 are directly connected, the engine 10 and the motor 20 are connected by an engine clutch 50 to transmit power, and an integrated starter-generator (ISG) 40 is mounted to the engine 10.
The motor 20 is used to start the vehicle upon initial start-up. When the vehicle reaches a predetermined speed, the generator, i.e., the ISG 40, starts the engine 10, and thus the outputs of the engine 10 and the driving motor 20 are used simultaneously.
In particular, the HEV may run in electric vehicle (EV) mode, which is directed to a purely electric vehicle mode using only the power of the motor 20, in hybrid electric vehicle (HEV) mode, which is an auxiliary mode using the rotational force of the motor 20 as an auxiliary power source with the use of the rotational force of the engine 10 as a main power source, and in regenerative braking (RB) mode, in which the braking energy or inertia energy of the vehicle produced by braking or during driving by inertia is recovered by power generation of the motor 20 and charged in a battery. Mode conversion from the EV mode to the HEV mode is performed during start-up of the engine 10 by the ISG 40.
In implementing the logic for the respective driving modes of such an HEV, the control logic for calculating a driver demand torque during initial acceleration of the vehicle should accurately reflect a driver's intention. If the driver's intention is not sufficiently reflected in calculating the driver demand torque, the vehicle may be driven against the driver's intention, which may result in a deterioration of driving performance and an increase in the risk of an accident.
Accordingly, the accuracy of the driver demand torque calculation is essentially required for the control logic implementation of the HEV.
However, in the event of failure of the ISG 40, the engine start-up is not easy. Although the engine start-up is available by clutch slip (friction start) in the above state, the driving performance may be deteriorated and further the acceleration performance of the vehicle may be deteriorated.
Accordingly, a separate control method for the mode conversion is required to utilize the engine power in the event of ISG failure without any problem.
As a conventional method for satisfying the above requirement, a method of starting up an engine (friction start) using friction of an engine clutch during driving has been proposed. However, the method only aims at starting up the engine by hydraulic profile modeling of the engine clutch, and drag generated by a transfer torque of the engine clutch during start-up is not compensated, thus deteriorating the driving performance and acceleration performance of the vehicle.
As another conventional method, a method of starting up an engine by a motor has been proposed, differently from the friction start for starting up the engine using the friction of the engine clutch during driving. However, the method has the same drawback in that the drag generated by the transfer torque of the clutch is not compensated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.